Nonlocal Plasmonic Response and Fano Resonances at Visible Frequencies in Sub-Nanometer Gap Coupling Regime

Plasmonic nanoassemblies endowed with ultrasmall interparticle gaps display a rich and unusual electromagnetic response both in near- and far-field readout. The properties of such systems can be mainly ascribed to the hybridization of localized surface plasmon modes, which in turn enables a higher degree of field confinement with respect to single particles. Pushing these plasmonic resonances toward stronger coupling regimes (i.e., smaller gaps) requires particular attention to the microscopic description of the system. In this article, we theoretically show that the appearing of nonlocal effects in the electromagnetic response of a plasmonic dimer in a strong coupling regime unveils a natural extension of its hybridization scheme. We demonstrate the arising of a Fano resonant interference in the visible range, resulting from the interaction of a hybrid quadrupolar mode and a highly localized gap mode.